The present invention relates generally to communication systems. More particularly, the present system relates to a digital subscriber line modem.
Explosive growth of the internet and the worldwide web drives increasing demands for faster communication data rates. In the corporate world, dedicated high-speed links (perhaps T1/E1 frame relays or OC1 ATM systems) from the company to an internet access provider satisfy current needs for highspeed access or data rates. Corporate users gain access to an internet router using a local area network (LAN). The router then connects to a high-speed link (e.g., T1/E1 lines). Unfortunately, residential users of the internet do not often have a high-speed link and must rely on standard analog or plain old telephone service (POTS) lines.
The increasing availability of information, data programs, entertainment, and other computer applications on the worldwide web and the internet strengthens the demand for high-speed access to the home. For example, designers of web technology constantly develop new ways to provide sensory experiences, including audio and video, to users of the web. Higher-speed modems will be required so the residential user can fully interact with future web and communication technologies.
Although designers of modems are continuously attempting to increase data rates, analog or POTS line modems can presently only reach data rates of up to 56 kilobits per second (Kbps). These conventional analog modems transmit and receive information on POTS subscriber lines through the public switched telephone network. The internet access provider is also coupled to the switched telephone network and transmits and receives information through it to the subscriber line.
Some residential users utilize integrated services digital network (ISDN) equipment and subscriptions to obtain up to 128 Kbps access or data rates the use of 2 B channels. ISDN equipment and subscriptions can, however, be expensive and require a dedicated subscriber line. Thus far, neither ISDN modems nor analog modems are capable of providing 256 Kbps or higher access between the home and the internet. Over one megabit per second (Mbps) data rates with analog modems or ISDN equipment do not seem feasible at this time.
A variety of communication technologies are competing to provide high-speed access to the home. For example, asymmetric digital subscriber lines (ADSL), cable modems, satellite broadcast, wireless LANs, and direct fiber connections to the home have all been suggested. Of these technologies, ADSL can utilize the POTS subscriber line (the wire currently being utilized for POTS) between the home user (the residence) and the telephone company (the central office).
ADSL networks and protocols were developed in the early 1990""s with the purpose of allowing telephone companies to provide video-on-demand service over the same wires which were being used to provide POTS. ADSL technologies include discrete multitone (DMT), carrier less amplitude and phase modulation (CAP), and other technologies. Although the video-on-demand market has been less than originally expected, telephone companies have recognized the potential application of ADSL technology for internet access and have begun limited offerings.
ADSL technology allows telephone companies to offer high-speed internet access. ADSL also permits telephone companies to remove internet traffic from the telephone switch network. Currently, telephone companies cannot significantly profit from internet traffic in the telephone switch network due to regulatory considerations. In contrast, ADSL allows the telephone company to charge a separate access fee for data services. The separate fee is not as restricted by regulatory considerations.
With reference to FIG. 1, a conventional asymmetric ADSL (ADSL) system 10 includes a copper twisted pair analog subscriber line 12, an ADSL modem 14, an ADSL modem 16, a band splitter 18, and a band splitter 20. Line 12 is a POTS local loop or wire connecting a central office 32 of the telephone company and a user""s residence 22.
ADSL modem 14 is located in user""s residence 22 and provides data to and from subscriber line 12. The data can be provided from line 12 through modem 14 to various equipment (not shown) coupled to modem 14. Equipment, such as, computers, network devices, servers, or other devices, can be attached to modem 14. Modem 14 communicates with a data network (not shown) coupled to modem 16 across line 12. ADSL modem 16 receives and transmits signals to and from line 12 to the data network, The data network can be coupled to other networks (not shown), including the internet.
At least one analog telephone 26, located in residence 22, can be coupled to subscriber line 12 through splitter 20 for communications across line 12 with telephone switch network 28. Telephone 26 and telephone switch network 28 (e.g., public-switched telephone (PST) network) are conventional systems well-known in the art. Alternatively, other analog equipment, such as, facsimile machines, POTS modems, answering machines, and other telephonic equipment, can be coupled to line 12 through splitter 20.
System 10 requires that band splitter 18 and band splitter 20 be utilized to separate higher frequency ADSL signals and lower frequency POTS signals. For example, when the user makes a call from residence 22 on telephone 26, lower frequency signals (under 4 kilohertz (kHz)) are provided through band splitter 20 to subscriber line 12 and through band splitter 18 to telephone switch network 28. Band splitter 18 prevents the lower frequency POTS signals from reaching ADSL modem 16. Similarly, band splitter 20 prevents any of the POTS signals from reaching modem 14.
ADSL modem 16 and ADSL modem 14 communicate higher frequency ADSL signals across subscriber line 12. The higher frequency ADSL signals are prevented from reaching telephone 26 and telephone switch network 28 by band splitters 20 and 18, respectively. Splitters 18 and 20 can be passive analog filters or other devices which separate lower frequency POTS signals (below 4 kHz) from higher frequency ADSL signals (above 25 kHz).
The separation of the POTS signals and ADSL signals by splitters 18 and 20 is necessary to preserve POTS voice and data traffic and ADSL data traffic. More particularly, splitters 18 and 20 can eliminate various effects associated with POTS equipment which may affect the transmission of ADSL signals on subscriber line 12. For example, the impedance of subscriber line 12 can vary greatly as at least one telephone 26 is placed on-hook or off-hook. Additionally, the changes in impedance of subscriber line 12 can change the ADSL channel characteristics associated with subscriber line 12. These changes in characteristics can be particularly destructive at the higher frequencies associated with ADSL signals (e.g., from 30 kHz to 1 megahertz (MHz) or more).
Additionally, splitters 18 and 20 isolate subscriber line wiring within residence 22. The impedance of such wiring is difficult to predict. Further still, the POTS equipment, such as, telephone 26, provides a source of noise and nonlinear distortion. Noise can be caused by POTS voice traffic (e.g., shouting, loud laughter, etc.) and by POTS protocol, such as, the ringing signal. The nonlinear distortion is due to the nonlinear devices included in conventional telephones. For example, transistor and diode circuits in telephone 26 can add nonlinear distortion and cause hard clipping of ADSL signals. Telephone 26 can further generate harmonics which can reach the frequency ranges associated with the ADSL signals. The nonlinear components can also demodulate ADSL signals to cause a hiss in the audio range which affects the POTS.
Conventional ADSL technology has several significant drawbacks. First, the costs associated with ADSL services can be quite large. Telephone companies incur costs related to central office equipment (ADSL modems and ADSL network equipment) and installation costs associated with the ADSL modems and network equipment. Residential users incur subscriber equipment costs (ADSL modems) and installation costs.
Installation costs are particularly expensive for the residential user because trained service personnel must travel to residence 22 to install band splitter 20 (FIG. 1). Although band splitter 18 must be installed at the central office, this cost is somewhat less because service personnel can install band splitter 18 within central office 32. Also, at office 32, splitter 18 can be included in ADSL modem 16. However, in residence 22, splitter 20 must be provided at the end of subscriber line 12.
Additionally, ADSL equipment for the residence, such as, modem 14, is expensive because the most complex component of modem 14 (e.g., the receiver) is located at residence 22 since high-speed transmissions are generally received within residence 22, and lower-speed transmissions are received by central office 32. In most internet applications, larger amounts of data are requested by the residential user rather than by the internet source. Receivers are typically much more complex than transmitters. These high-speed receivers often receive data at rates of over 6 Mbps.
ADSL equipment can be subject to cross-talk noise from other subscriber lines situated adjacent to subscriber line 12. For example, subscriber lines are often provided in a closely contained bundle. The close containment can cause cross-talk from other subscriber lines to be placed on subscriber line 12. Modem 14 must compensate for cross-talk noise.
U.S. application Ser. No. 08/943,484, entitled, xe2x80x9cSplitterless Digital Subscriber Line Communication system,xe2x80x9d filed on Oct. 3, 1997, by Henderson, et al. describes a digital subscriber line (DSL) communication system which does not require the use of a splitter in the residence. The splitterless communication system allows a DSL modem to be connected directly to the subscriber line similar to the use of a conventional analog modem. The DSL modem used in the splitterless communication system is less expensive and does not utilize a considerably expensive high-speed receiver which operates at data rates over 2 Mbps.
As mentioned above, however, the presence of transistor and diode circuits in telephones can add non-linear distortion and cause hard clipping of ADSL signals. Non-linear components can also demodulate ADSL signals to cause a hiss in the audio range. The demodulation, distortion, and hard clipping which in conventional ADSL systems is shielded to a large degree by band splitter 20 can affect splitterless ADSL systems much more severely, since three is no band splitter at the user""s residence.
Thus, there is a need for a power cutback level in splitterless DSL systems that achieves acceptable levels of noise reduction. Further, there is a need for reducing power as much as possible while preserving the signal to noise ratio at an acceptable level. Further still, there is a need to counter the demodulation effects of non-linear telephone devices on the telephone line.
One embodiment of the invention relates to a splitterless digital subscriber line modem adapted to be coupled to a subscriber line including a sending end and a receiving end, the modem being capable of simultaneous access to the subscriber line with other telephone equipment operating in a frequency band below four kilohertz. The modem includes a data terminal and a control circuit. The data terminal couples the modem to the subscriber line. The control circuit is coupled to the data terminal and receives and transmits signals to and from the data terminal. The control circuit utilizes line coding techniques to measure signal and noise at the receiving end and adjusts amplitude of the signal at the sending end in response to the signal and the noise whereby power of the signal is optimized.
Another embodiment of the invention relates to a communication system for use with a subscriber line. The communication system includes a user splitterless digital subscriber line modem, a splitter, and an office digital subscriber line modem. The user splitterless digital subscriber line modem is located at a office site and is coupled directly to the subscriber line. The modem receives downstream signals from the subscriber line and transmits upstream signals to the subscriber line. The office digital subscriber line modem utilizes line coding techniques to measure signal and noise at the office site and transmits control signals to the user splitterless digital subscriber line modem allowing it to adjust amplitude of the signal in response to the signal and the noise whereby power of the signal is optimized. The splitter is located remote from the user site and has a signal terminal, a lower frequency path terminal, and a higher-frequency path terminal. The signal terminal is coupled to the subscriber line. The lower frequency path terminal is coupled to a switched telephone network. The office digital subscriber line modem is coupled to the higher frequency path terminal. The office digital subscriber transmits the down stream signals to the subscriber line to the splitter and receives the upstream signals from the subscriber line through the splitter.
Another embodiment of the invention relates to a method of optimizing total transmitted power over a subscriber line including a sending end and receiving end in a splitterless asynchronous digital subscriber line (ADSL) system. The method includes utilizing line coding techniques to measure signal and noise at the receiving end and adjusting signal amplitude at the sending end based on signal and noise measured at the receiving end.